The present invention relates to a charging apparatus including a charging member for charging a member to be charged, such as a photosensitive member or a dielectric body, while forming facilitator particles. The present invention also relates to a process cartridge and an image forming apparatus, such as an electrophotographic apparatus or an electrostatic recording apparatus, which includes the charging apparatus.
A contact-type charging apparatus using electroconductive particles has been disclosed, e.g., in U.S. Pat. Nos. 6,081,681; 6,128,456: and 6,134,407.
In this charging apparatus, a member to be charged is caused to contact a charging member generally comprising an electroconductive elastic foam roller with a speed difference therebetween at a charge-contact portion (charging nip) where electroconductive particles are disposed at least in contact with both the member to be charged and the charging member, while applying a charging bias to the charging member. As a result, a direct charge-injection mechanism is dominant rather than an electrical discharge-based charging mechanism.
(1) Electrical Discharge-based Charging Mechanism
This charging mechanism is a mechanism in which the surface of a member to be charged is charged by electrical discharge which occurs across a microscopic gap between a contact-type charging member and the member to be charged.
In the case of the electrical discharge-based charging mechanism, a certain discharge threshold is present between the contact-type charging member and the member to be charged, thus requiring application of a voltage to the contact-type charging member with a value larger than a charging potential therefor. Further, in principle, it is impossible to avoid generating by-products of electrical discharge, although the amount thereof is considerably small in comparison with the case of a non-contact-type charging apparatus, such as a corona discharger. Accordingly, the contact-type charging member cannot completely eliminate the problems caused by active ions, such as ionized ozone.
For example, a roller-charging scheme using an electroconductive roller (charging roller) as the contact-type charging member is preferred in terms of stability and is used widely, but in this roller-charging scheme, the electrical discharge-based charging mechanism is dominant in this charging scheme.
(2) Direct Charge-injection Mechanism
This is a mechanism in which the surface of a member to be charged is charged by direct injection of electrical charge from a contact-type charging member to the member to be charged. More specifically, a contact-type charging member with medium electrical resistance is placed in contact with the surface of a member to be charged to directly inject electrical charge into the surface portion of the member to be charged, without relying on an electrical-discharge phenomenon, i.e., without principally using an electrical-discharge mechanism. Therefore, even if the value of the voltage applied to the contact-type charging member is below the discharge-threshold value, the member to be charged can be charged to a voltage level which is substantially the same as the level of the voltage applied to the contact-type charging member. This direct injection-charging mechanism does not suffer from the problems caused by the by-product of electrical discharge since it is not accompanied by the occurrence of ions.
As described above, in the contact-type charging apparatus using electroconductive particles, the direct charge-injection mechanism is dominant. The electroconductive particles are particles for charge facilitation. As the electroconductive particles, it is possible to use various electroconductive particles including electroconductive fine particles of metal oxide, such as zinc oxide, other inorganic fine particles, mixtures thereof with an organic compound, or the like.
By the presence of the electroconductive particles, the contact-type charging member can be in contact with the member to be charged at a contact-charge portion therebetween with a difference in speed with the member to be charged. At the same time, the charging member intimately contacts the member to be charged via the electroconductive particles. In other words, the electroconductive particles present at the contact-charge portion are frictionally disposed in close contact with the surface of the member to be charged, thus effecting direct injection of electrical charge into the member to be charged. That is, charging of the member to be charged by the contact-type charging member supplied with a charging bias can make the direct charge-injection mechanism dominant.
As a result, in the direct charge-injection mechanism, it is possible to obtain a high charge efficiency that has not been attained by the aforementioned roller charging, and so forth. Further, it is possible to impart an electrical potential substantially equal to a voltage applied to the member to be charged, thus realizing ozone-less direct charge injection with a simple structure at a low applied voltage. As an embodiment, in an electrophotographic image forming apparatus or an electrostatic recording-type image forming apparatus, the contact-type charging member is effective as charging means for uniformly charging an image bearing member, such as an electrophotographic photosensitive member or an electrostatic recording dielectric member, to a predetermined polarity and potential level through direct charge injection.
Further, in recent years, an image forming apparatus is increasingly required to be a cleanerless system producing no waste toner from an ecological standpoint. In the aforementioned direct charge-injection mechanism using electroconductive particles, it is possible to effect uniform charging by the use of a cleanerless apparatus.
In a specific embodiment, electroconductive particles are mixed with a developer and at a developing position, are supplied together with a toner from a developing apparatus to the surface of an image bearing member, such as an electrophotographic photosensitive member, as a member to be charged. At a transfer position, only the toner is principally transferred onto a transfer medium and, the electroconductive particles are supplied to the charge-contact portion, thus allowing uniform charging based on injection charging in the cleanerless apparatus.
However, in such a direct charge-injection mechanism using electroconductive particles, the following problem arises.
More specifically, electroconductive particles on the contact-type charging member are gradually detached therefrom to the surface of the member to be charged. In the aforementioned cleanerless image forming apparatus, even if the contact-type charging member is replenished with electroconductive particles from the developing apparatus via the image bearing member surface to maintain a direct charge-injection state, with the use of the image forming apparatus, the detachment amount of electroconductive particles detaching from the surface of the contact-type charging member and the replenishing amount of electroconductive particles replenished from the developing apparatus to the contact-type charging member via the image bearing member surface become out of balance, thus resulting in an insufficient amount of electroconductive particles required for adequate charging on the contact-type charging member in some cases. In such cases, the charging performance is lowered to cause image failure in an image forming apparatus. This phenomenon is liable to occur by the use of the cleanerless image forming apparatus for a long period.
An object of the present invention is to provide a charging apparatus, a process cartridge and an image forming apparatus, in which electroconductive particles are liable to remain on a charging member.
Another object of the present invention is to provide a charging apparatus, a process cartridge and an image forming apparatus, which are capable of suppressing movement of electroconductive particles from a charging member to a member to be charged.
Another object of the present invention is to provide a charging apparatus, a process cartridge and an image forming apparatus, which are capable of providing a stable charging performance of a charging member against a member to be charged.
Another object of the present invention is to provide a charging apparatus, a process cartridge and an image forming apparatus, which are suitable for the use of a cleanerless scheme which is liable to cause a soiled state of a charging member with a transfer residual toner as a charge-inhibition factor.
According to the present invention, there is provided a charging apparatus comprising:
a charging member for charging a member to be charged while forming a nip with the member to be charged, and
electroconductive particles disposed in the nip, wherein
the charging member is supplied with a voltage of a certain polarity, a surface of the member to be charged is triboelectrically charged by a surface of the charging member so as to have a polarity opposite from that of the voltage supplied to the charging member, and the electroconductive particles are triboelectrically charged in the nip so as to have a polarity opposite from that of the voltage.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.